This invention relates to a process for the preparation of highly-branched polyols by polymerisation of glycidol in the presence of a hydrogen-active starter compound with basic catalysis.
Branched polyols based on glycidol are conventionally prepared by reacting glycidol with a hydroxyl-containing compound, for example, glycerol, in the presence of inorganic (JP-A 61-43627) or organic (JP-A 58-198429) acids as catalysts. The polymers thus obtained generally have a low degree of polymerisation. The polymerisation of glycidol to products of higher molecular weight which have a narrow molar-mass distribution and complete incorporation of initiators cannot be achieved by cationic catalysis, because of the competing cyclisation reactions (Macromolecules, 27 (1994) 320; Macromol Chem. Phys. 196 (1995) 1963). Existing processes using basic catalysis (EP-A 116 978; J. Polym. Sci., 23(4) (1985) 915), likewise do not lead to colourless products free of by-products and having a narrow molar-mass distribution and complete incorporation of initiators. A secondary reaction of significance here is in particular the cyclisation as a result of the autopolymerisation of glycidol.
Accordingly, the object of the present invention was to find a process for the preparation of highly-branched polyols based on glycidol whereby the problems described above are avoided.
Surprisingly, it has now been found that it is possible to prepare colourless, highly-branched polyols based on glycidol which are narrowly distributed and have a defined structure, if a dilute solution containing glycidol is added to a hydrogen-active starter compound, with basic catalysis, the solvent used for the dilution being continuously distilled off. In this connection, xe2x80x9cdefined structurexe2x80x9d means that each molecule possesses the initiator (hydrogen-active starter compound) as the core unit and the degree of polymerisation can be controlled via the monomer/initiator ratio.
The invention provides a process for the preparation of highly-branched polyols based on glycidol which have a defined structure, which is characterised in that a dilute solution containing glycidol is added to a hydrogen-active starter compound, in the presence of a basic catalyst, the solvent used for the dilution of the monomer being continuously distilled off.
As a result of the preferential opening of the epoxide ring at the unsubstituted end where basic catalysis is used, a secondary alkoxide is first of all produced, which, however, in consequence of the basic catalysis, is in rapid exchange with the primary alkoxide. The rapid proton exchange equilibrium ensures that all hydroxyl groups present in the system are active as regards polymerisation and that there is a resulting development of branching.